I had a block that I bought off ebay, and had subsequently rebuilt an engine with it.... and then it subsequently died with some very loud rod knocking...

so....

I pulled the oil pan, checked all the rod bearings..... and... well, let's just say that there were NO bearings on the #2 cylinder..... there is a little heat discoloration, but, the surface of the rod and the crankshaft are still good, no significant wear or marring of the surface....

so... my question is...

would it be safe to just buy a new set of bearings for that one cylinder, (the rest of the bearings are all within spec) or, should I buy a whole new set?

yeah buy new, honestly id get the heat diss-coloration checked by someone who may know , even though it may look ok, it could be worse than you think. ive seen engines blow with minimal heat damage and they say to send it out to be machined.